Rat and mouse cells were separated with an improved nonsynchronous flow- through coil planet centrifuge (CPC). Peritoneal cells were suspended at the concentration of 2-3 x 10 7 cells/ml in conditioned RPMI 1640, supplemented with 50% heat inactivated FCS and 0.32% sodium citrate. In each separation 3ml of cell suspensions were loaded into the coiled column and elutriated at 4 degrees C. Several conditions including the centrifugal force, revolution/rotation ratio, density of separation media, flow speed, designs of both coiled column and flow tubes, etc. were examined and optimized for mast cell purification. Rat mast cells were separated at the purity of 99.1%, with an average yield of 42%. Over 90% pure mouse mast cells were harvested, despite a very low population of mast cells available in murine peritoneal cells. Purified cells were biologically intact and showed little damage.